1. Field of the Invention
This invention relates to an optical beam scanning apparatus for forming an image through scanning of an optical beam.
2. Description of the Prior Art
An optical beam scanning apparatus for forming an image through scanning of an optical beam is known. Such a prior art optical beam scanning apparatus comprises a laser diode for generating a laser beam, a polygon mirror unit for deflecting the laser beam in a main scanning direction, a f.theta. lens for forming a spot on a recording medium with a constant speed in the main scanning direction and with the spot focused on the recording medium.
FIG. 27 is a schematic plan view of a prior art optical beam scanning apparatus. This prior art optical beam scanning apparatus comprises a laser diode 1 for generating a laser beam, a collimeter lens 2 for collimating the laser beam, a cylindrical lens 3 for focusing the laser beam on a mirror surface of the polygon mirror only in the sub-scanning direction, a polygon mirror unit 4 for deflecting the laser beam in a main scanning direction, a f.theta. lens system 6 for forming a spot on a recording medium with a constant speed in the main scanning direction and with the spot focused on the recording medium. FIG. 28 is a graphic diagram of an aberration characteristic of a prior art general lens and a prior art f.theta. lens. In the general lens, a height of a formed image from an optical axis varies with an incident angle of the image non-linearly. The f.theta. lens compensates this change linearly.
Assuming the laser beam is directed by the polygon mirror with a deflection angle .theta. and emitted at the final surface of the f.theta. lens system with an emission angle .alpha., the scanning speed on the recording medium is constant, that is, f.multidot.d.theta./dt and the emission angle .alpha. is less than the deflection angle .theta..